1. Field of the Invention
The invention relates to a circuit-incorporating light receiving device which incorporates a circuit for processing a photoelectric conversion signal, and particularly to a structure of a split photodiode, in which a light receiving region is split into a plurality of light detecting portions, which can improve the response speed, and which can reduce the deterioration of properties due to misalignment of optical split positions of the light detecting portions and structural split positions.
2. Description of the Related Art
Conventionally, for example, such a split photodiode is used as a signal detecting device for an optical pickup.
Recently, with the achievement of miniaturization and high performance of an optical disk apparatus, the reduction in size and weight of an optical pickup has become an important issue to be realized. In order to realize this, an optical module has been proposed which performs the functions of generating a tracking beam, conducting optical branching, and generating an error signal all in one hologram device; a laser diode, a photodiode, etc. are housed in one package (not shown); and the hologram device is disposed on the upper face of the package.
FIG. 3 diagrammatically shows the configuration of the optical system of the optical pickup. The principle of the signal detection in the optical system will be described briefly. Light emitted from a laser diode LD is split into three light beams, i.e., two subbeams for tracking and one main beam for reading a data signal. The light from the laser diode is split by a diffraction grating 30 which is disposed on the back face of the hologram device 31. The diffraction grating 30 is for generating a tracking beam.
The light beams which have passed as zero-order light through the hologram device 31 on the upper face of the package are converted into parallel light beams by a collimator lens 32, and then focused on a disk 34 by an objective lens 33. Reflected light beams which have undergone modulation by pits on the disk 34 pass through the objective lens 33 and the collimator lens 32, and are then diffracted by the hologram device 31. The diffracted beams are guided as first-order light onto a five-split photodiode PD having five split light detecting portions (hereinafter, often referred to as "light detecting photodiode portions") D1 to D5.
The hologram device 31 has two regions having different diffraction periods. Among reflected light of the main beam, a portion entering one of the regions is focused on an isolation line by which the light detecting portions D2 and D3 are split from each other, and a portion entering the other region is focused on the light detecting portion D4. The reflected light of each subbeam is focused on respective ones of the light detecting portions D1 and D5 by hologram device 31. Depending on the change of the distance between the hologram device 31 and the disk 34, the position of the reflected light of the main beam on the photodiode PD is moved in the direction along which the light detecting photodiode portions D2 and D3 are aligned, so that, when the main beam is focused on the disk, the reflected light of the main beam enters the isolation region between the light detecting photodiode portions D2 and D3.
When the outputs of the five-split photodiode PD respectively corresponding to the light detecting portions D1 to D5 are indicated by S1 to S5, the focus error signal FES is given by the following equation: EQU FES=S2-S3
On the other hand, the tracking error is detected by the so-called three-beam method. Since the two tracking subbeams are focused on the light detecting portions D1 and D5, respectively, the tracking error signal TES is given by the following equation: EQU TES=S1-S5
When the error signal TES is 0, it means that the main beam is correctly positioned on a track to be irradiated. The reproduction signal RF is given as the total sum of the outputs of the light detecting portions D2 to D4 receiving the reflected light of the main beam, or as follows: EQU RF=S2+S3+S4
FIG. 4 is a plan view showing the structure of the five-split photodiode PD which is incorporated in the configuration of the above-mentioned optical system. The shape of the five-split photodiode depends on the optical system. In this example, the light detecting portions of the photodiode have a lengthways elongated shape. This shape is determined by the reason described below.
The laser diode LD and the photodiode PD constituting the optical system are incorporated in one package, and the hologram device 31 is adhered to the upper face of the package. The positions of the laser diode and the photodiode are caused to be varied in the processes of positioning them. Moreover, the oscillation wavelength of the laser diode LD is different between devices, and changes due to temperature variations. Variations produced in the processes of positioning the laser diode and the photodiode, and variations of the oscillation wavelength cause the angle of diffraction of diffracted light to be changed. Accordingly, the light receiving face of the photodiode PD is required to have a larger dimension in the Y-direction as shown in FIG. 4, or the direction along which the incidence position of reflected light on the photodiode is changed when the angle of diffraction is changed.
The dimension in the X-direction of the light receiving face which is perpendicular to Y-direction is not affected by the change of the angle of diffraction which is caused by variations of the oscillation wavelength of the laser diode between devices and changes in oscillation wavelength owing to the temperature variation. Since variations produced in the processes of positioning the laser diode and the photodiode can be adjusted by rotating the hologram device 31 when the device is adhered to the upper face of the package, the dimension of the light receiving face in the X-direction is not required to have a large value. Conversely, in the X-direction of the light receiving face, the adjustment in the process of incorporating the optical pickup in an optical disk apparatus is difficult when the three beams arranged in the X-direction are separated from each other. In the photodiode, therefore, the widths of the light detecting portions D1 to D5, and those of the isolation regions between the light detecting portions must be narrowed.
For the above-mentioned reason, the light detecting portions of the photodiode PD naturally have a lengthways elongated shape as shown in FIG. 4.
FIG. 5 shows the cross-sectional structure of the portion of the photodiode along line a-a' shown in FIG. 4. In FIG. 4, 201 designates a five-split photodiode for detecting light which has conventionally been used, 202 designates an anode electrode which is common to light detecting photodiode portions D1 to D5, and 203a to 203e designate cathode electrodes respectively corresponding to the light detecting photodiode portions D1 to D5. In the figure, components formed in steps conducted after the metallization step, such as multi-layer conductors, a passivation film, and the like are not shown.
The photodiode is produced in the following manner. First, P-type buried diffusion layers 2 are formed in regions on a P-type semiconductor substrate 1 which will be formed as the isolation regions used for splitting the light detecting portion (FIG. 6A).
Next, as shown in FIG. 6B, an N-type epitaxial layer 4 is formed on the entire surface of the P-type semiconductor substrate 1. P-type isolation diffusion layers 5 elongating from the surface of the N-type epitaxial layer 4 to the portions corresponding to the P-type buried diffusion layers 2 are then formed so that the light detecting portions D1 to D5 which are electrically isolated from each other are formed.
A P-type diffusion layer 6 is then formed on the surface of the N-type epitaxial layer 4, and also on the surfaces of the P-type isolation diffusion layers 5 which will be formed as the isolation regions for the light detecting portions D1 to D5 (FIG. 6C).
As shown in FIG. 6D, in an oxide film 7 which is formed on the surface in the step of forming the P-type diffusion layer 6, the portion corresponding to the light receiving region of the surface of the P-type diffusion layer 6 is removed. Then, a nitride film 8 is formed on the entire surface. In order that the nitride film 8 will function as an anti-reflection film, the thickness of the nitride film 8 is selected so as to conform to the wavelength of the laser diode.
Next, openings are formed in the nitride film 8 and the oxide film 7 to form electrode windows. Electrode conductors 9a are formed, and at the same time metal films 9 are formed on portions of the surface of the nitride film 8 which are not irradiated with signal light, thereby obtaining the five-split photodiode having the structure shown in FIG. 5. A signal processing circuit (not shown) is formed on the semiconductor substrate 1 by a usual bipolar IC process.
In the thus configured five-split photodiode PD, the PN junctions of the isolation regions for the light detecting portions D1 to D5 are covered by the P-type diffusion layer 6. Even when the nitride film 8 is formed directly on the surface of the photodiode, problems such as increased junction leakage do not arise. In the isolation region between the light detecting portions D2 and D3 of the photodiode on which a focused beam actually impinges, therefore, the reflection of the focused beam at the light receiving face is suppressed to a low level by the nitride film 8. Consequently, the high sensitivity of the photodiode can be realized.
Since the metal film 9 is formed in the portions which are not irradiated with signal light, in the example, the portions between the light detecting portions D1 and D2, and D3 and D5, the photodiode is hardly affected by stray light or the like so that the S/N ratio of the photodiode is improved.
However, particularly, the light detecting portions D2, D3, and D4 which process the reproduction signal RF are required to operate at a high speed. It was found that, in the light detecting portions D2 and D3, the cutoff frequency obtained when the isolation regions of these portions are irradiated with a light beam is lower than that obtained when the center region of the respective light detecting portions is irradiated with a light beam. FIG. 7 shows the experimental results.
The state where the isolation region of the adjacent light detecting portions was irradiated with a light beam was analyzed by using a device simulation system. As a result, it was found that, under this state, optical carriers detour around the P-type buried diffusion layer 2 of the isolation region and reach the junction of the N-type epitaxial layer 4 and the P-type semiconductor substrate 1 so that optical carriers are caused to move a longer distance by diffusion, thereby causing the cutoff frequency to be lowered.
In FIG. 8, directions of currents in the P-type buried diffusion layer 2 of the isolation region and in the vicinity thereof are indicated by arrows. Electrons which function as optical carriers move in directions opposite to those of the arrows.
FIG. 9 shows the potential distribution in the depth direction of the isolation diffusion layer 5 of the isolation region. As seen from the figure, the potential distribution operates as a potential barrier against electrons which function as optical carriers in the substrate and directed toward the surface of the substrate. Therefore, it was found that optical carriers detour around the P-type buried diffusion layer 2. The detour causes the moving distance of electrons to be the order of 10 micrometers.